High performance fibres composite sheet

ABSTRACT

The invention relates to a method for manufacturing a composite sheet comprising high performance polyethylene fibres and a polymeric resin comprising the steps of assembling HPPE fibres to a sheet, applying an aqueous suspension of a polymeric resin to the HPPE fibres, partially drying the aqueous suspension, optionally applying a temperature and/or a pressure treatment to the composite sheet wherein the polymeric resin is a homopolymer or copolymer of ethylene and/or propylene. The invention further relates to composite sheets obtainable by said method and articles comprising the composite sheet such as helmets, radomes or a tarpaulins.

The present invention concerns a method for producing a composite sheetcomprising high performance polyethylene fibres and a polymeric resinand such composite sheet. These composite sheets are amongst othersespecially adapted to facilitate the manufacture of ballistic resistantarticles, amongst which soft ballistic articles for example for vestsand moulded ballistic articles for example for vehicle protection,combat helmets, or inserts.

Composite materials comprising high performance polyethylene fibres anda polymeric resin as a matrix material are known from U.S. Pat. No.4,623,574, U.S. Pat. No. 5,766725, U.S. Pat. No. 7,211,291 and U.S. Pat.No. 8,999,866. U.S. Pat. No. 4,623,574 discloses the manufacture ofballistic resistant sheets by cross plying and stacking a plurality ofmonolayers comprising unidirectionally aligned extended chainpolyethylene fibers and a matrix material, followed by pressing thecross-plied and stacked monolayers into a sheet. Example 1 of U.S. Pat.No. 4,623,574 mentions the production of unidirectional monolayers byhelically wrapping polyethylene fibers side-by-side on a drum winderwhereby a Kraton D1107 solution is used to coat the unidirectionallyaligned fibers. A plurality of the thus obtained unidirectionalmonolayers was stacked whereby the fiber direction in a monolayer isperpendicular to the fiber direction in an adjacent monolayer. Theobtained stack was pressed, followed by cooling to provide a moldedballistic article.

There is continuous drive towards improved ballistic resistant articlesand the present inventors have surprisingly found a method to produceballistic resistant sheet that enables the manufacture of soft ballisticsheets or ballistic resistant moulded articles with improved ballisticresistance properties. Such improved ballistic resistance properties mayfor example be expressed in a reduction of the delamination of compositematerials upon use, resulting in increased protection by the ballisticresistant products. Preferably the ballistic protection may relate tothe bullet stopping characteristics, the reduction of trauma or backface deformation or a reduction of material deterioration bydelamination upon use.

It is the aim of the present invention to provide a manufacturingprocess and the thereby obtainable composite material that at leastpartly overcome the above mentioned problems.

The present invention solves this need by applying an aqueous suspensioncomprising polymeric resin to the high performance polyethylene (HPPE)fibres before, during or after the step of assembling said HPPE fibresto an assembly, at least partially drying the aqueous suspension of thepolymeric resin applied to the HPPE fibres to obtain a composite sheet,optionally applying a temperature in the range from the meltingtemperature of the resin to 153° C. to the assembly before, duringand/or after at least partially drying the suspension to at leastpartially melt the polymeric resin and optionally applying a pressure tothe at least partially dried composite sheet before, during and/or afterthe temperature treatment to at least partially compact the compositesheet, wherein the polymeric resin is a homopolymer or copolymer ofethylene and/or propylene and wherein said polymeric resin has a densityas measured according to ISO1183 in the range from 860 to 930 kg/m³, apeak melting temperature in the range from 40 to 140° C. and a heat offusion of at least 5 J/g.

It has unexpectedly been found that the composite sheet manufacturedaccording to the method of the present invention may show an improveddelamination behaviour. Said improvement was demonstrated for both softballistic and moulded ballistic articles by an increased peel strengthof the composite sheet and a reduced back face deformation of mouldedarticles comprising a stack of the composite sheets respectively.

By fibre is herein understood an elongated body, the length dimension ofwhich is much greater than the transverse dimensions of width andthickness. Accordingly, the term fiber includes filament, ribbon, strip,band, tape, and the like having regular or irregular cross-sections. Thefiber may have continuous lengths, known in the art as filament orcontinuous filament, or discontinuous lengths, known in the art asstaple fibers. A yarn for the purpose of the invention is an elongatedbody containing many individual fibers. By individual fiber is hereinunderstood the fiber as such. Preferably the HPPE fibres of the presentinvention are HPPE tapes, HPPE filaments or HPPE staple fibres.

In the context of the present invention HPPE fibres are understood to bepolyethylene fibres with improved mechanical properties such as tensilestrength, abrasion resistance, cut resistance or the like. In apreferred embodiment high performance polyethylene fibres arepolyethylene fibres with a tensile strength of at least 1.0 N/tex, morepreferably at least 1.5 N/tex, more preferably at least 1.8 N/tex, evenmore preferably at least 2.5 N/tex and most preferably at least 3.5N/tex. Preferred polyethylene is high molecular weight (HMWPE) orultrahigh molecular weight polyethylene (UHMWPE). Best results wereobtained when the high performance polyethylene fibers compriseultra-high molecular weight polyethylene (UHMWPE) and have a tenacity ofat least 2.0 N/tex, more preferably at least 3.0 N/tex.

Preferably the composite sheet of the present invention comprises HPPEfibres comprising high molecular weight polyethylene (HMWPE) orultra-high molecular weight polyethylene (UHMWPE) or a combinationthereof, preferably the HPPE fibres substantially consist of HMWPEand/or UHMWPE. The inventors observed that for HMWPE and UHMWPE the bestballistic performances could be achieved.

In the context of the present invention the expression ‘substantiallyconsisting of’ has the meaning of ‘may comprise a minor amount offurther species’ wherein minor is up to 5 wt %, preferably of up to 2 wt% of said further species or in other words ‘comprising more than 95 wt% of’ preferably ‘comprising more than 98 wt % of° HMWPE and/or UHMWPE.

In the context of the present invention the polyethylene (PE) may belinear or branched, whereby linear polyethylene is preferred. Linearpolyethylene is herein understood to mean polyethylene with less than 1side chain per 100 carbon atoms, and preferably with less than 1 sidechain per 300 carbon atoms; a side chain or branch generally containingat least 10 carbon atoms. Side chains may suitably be measured by FTIR.The linear polyethylene may further contain up to 5 mol % of one or moreother alkenes that are copolymerisable therewith, such as propene,1-butene, 1-pentene, 4-methylpentene, 1-hexene and/or 1-octene.

The PE is preferably of high molecular weight with an intrinsicviscosity (IV) of at least 2 dl/g; more preferably of at least 4 dl/g,most preferably of at least 8 dl/g. Such polyethylene with IV exceeding4 dl/g are also referred to as ultra-high molecular weight polyethylene(UHMWPE). Intrinsic viscosity is a measure for molecular weight that canmore easily be determined than actual molar mass parameters like numberand weigh average molecular weights (Mn and Mw).

The HPPE fibres used in the method according to the invention may beobtained by various processes, for example by a melt spinning process, agel spinning process or a solid state powder compaction process.

One preferred method for the production of the fibres is a solid statepowder process comprising the feeding the polyethylene as a powderbetween a combination of endless belts, compression-molding thepolymeric powder at a temperature below the melting point thereof androlling the resultant compression-molded polymer followed by solid statedrawing. Such a method is for instance described in U.S. Pat. No.5,091,133, which is incorporated herein by reference. If desired, priorto feeding and compression-molding the polymer powder, the polymerpowder may be mixed with a suitable liquid compound having a boilingpoint higher than the melting point of said polymer. Compression moldingmay also be carried out by temporarily retaining the polymer powderbetween the endless belts while conveying them. This may for instance bedone by providing pressing platens and/or rollers in connection with theendless belts.

Another preferred method for the production of the fibres used in theinvention comprises feeding the polyethylene to an extruder, extruding amolded article at a temperature above the melting point thereof anddrawing the extruded fibres below its melting temperature. If desired,prior to feeding the polymer to the extruder, the polymer may be mixedwith a suitable liquid compound, for instance to form a gel, such as ispreferably the case when using ultra high molecular weight polyethylene.

In yet another method the fibres used in the invention are prepared by agel spinning process. A suitable gel spinning process is described infor example GB-A-2042414, GB-A-2051667, EP 0205960 A and WO 01/73173 A1.In short, the gel spinning process comprises preparing a solution of apolyethylene of high intrinsic viscosity, extruding the solution into asolution-fibre at a temperature above the dissolving temperature,cooling down the solution-fibre below the gelling temperature, therebyat least partly gelling the polyethylene of the fibre, and drawing thefibre before, during and/or after at least partial removal of thesolvent.

In the described methods to prepare HPPE fibres drawing, preferablyuniaxial drawing, of the produced fibres may be carried out by meansknown in the art. Such means comprise extrusion stretching and tensilestretching on suitable drawing units. To attain increased mechanicaltensile strength and stiffness, drawing may be carried out in multiplesteps.

In case of the preferred UHMWPE fibres, drawing is typically carried outuniaxially in a number of drawing steps. The first drawing step may forinstance comprise drawing to a stretch factor (also called draw ratio)of at least 1.5, preferably at least 3.0. Multiple drawing may typicallyresult in a stretch factor of up to 9 for drawing temperatures up to120° C., a stretch factor of up to 25 for drawing temperatures up to140° C., and a stretch factor of 50 or above for drawing temperatures upto and above 150° C. By multiple drawing at increasing temperatures,stretch factors of about 50 and more may be reached. This results inHPPE fibres, whereby for ultrahigh molecular weight polyethylene,tensile strengths of 1.5 N/tex to 3 N/tex and more may be obtained.

In one process step of the present invention an aqueous suspension isapplied to the HPPE fibres. Such application of suspension takes placebefore, during or after the fibres are assembled into a sheet. Byaqueous suspension is understood that particles of the polymeric resinare suspended in water acting as non-solvent. The concentration of thepolymeric resin may widely vary and is mainly limited by the capabilityto formulate a stable suspension of the resin in water. A typical rangeof concentration is between 2 and 80 wt % of polymeric resin in water,whereby the weight percentage is the weight of polymeric resin in thetotal weight of aqueous suspension. Preferred concentration are between4 and 60 wt %, more preferably between 5 and 50 wt %, most preferablybetween 6 and 40 wt %. Further preferred concentrations of the polymericresin in the dispersion is at least 15 wt %, preferably at least 18 wt %and even more preferably at least 20 wt %. In another preferredembodiment the concentration of the polymeric resin in the aqueousdispersion is between 10 and 50 wt %, preferably between 15 and 40 wt %,most preferably between 18 wt % and 30 wt %. Such preferred higherconcentrations of polymeric resin may have the advantage of a providinglengthy body with higher concentration while reducing the time andenergy required for the removal of the water from the lengthy body. Thesuspension may further comprise additives such as ionic or non-ionicsurfactants, tackyfying resins, stabilizers, anti-oxidants, colorants orother additives modifying the properties of the suspension, the resinand or the prepared composite sheet. Preferably the suspension issubstantially free of additives that may act as solvents for thepolymeric resin. Such suspension may also be referred to assolvent-free. By solvent is herein understood a liquid in which at roomtemperature the polymeric resin is soluble in an amount of more than 1wt % whereas a non-solvent is understood a liquid in which at roomtemperature the polymeric resin is soluble in an amount of less than 0.1wt %.

The polymeric resin present in the applied aqueous suspension andultimately present in the obtained composite sheet of the presentinvention is a homopolymer or copolymer of ethylene and/or propylene,also referred to as polyethylene, polypropylene or copolymers thereof,in the context of the present invention also referred to as polyolefinresin. It may comprise the various forms of polyethylene,ethylene-propylene co-polymers, other ethylene copolymers withco-monomers such as 1-butene, isobutylene, as well as with hetero atomcontaining monomers such as acrylic acid, methacrylic acid, vinylacetate, maleic anhydride, ethyl acrylate, methyl acrylate; generallyα-olefin and cyclic olefin homopolymers and copolymers, or blendsthereof. Preferably the polymeric resin is a copolymer of ethylene orpropylene which may contain as co-monomers one or more olefins having 2to 12 C-atoms, in particular ethylene, propylene, isobutene, 1-butene,1-hexene, 4-methyl-1-pentene, 1-octene, acrylic acid, methacrylic acidand vinyl acetate. In the absence of co-monomer in the polymeric resin,a wide variety of polyethylene or polypropylene may be used amongstwhich linear low density polyethylene (LLDPE), very low densitypolyethylene (VLDPE), low density polyethylene (LDPE), isotacticpolypropylene, atactic polypropylene, syndiotactic polypropylene orblends thereof.

Furthermore, the polymeric resin may be a functionalized polyethylene orpolypropylene or copolymers thereof or alternatively the polymeric resinmay comprise a functionalized polymer. Such functionalized polymers areoften referred to as functional copolymers or grafted polymers, wherebythe grafting refers to the chemical modification of the polymer backbonemainly with ethylenically unsaturated monomers comprising heteroatomsand whereas functional copolymers refer to the copolymerization ofethylene or propylene with ethylenically unsaturated monomers.Preferably the ethylenically unsaturated monomer comprises oxygen and/ornitrogen atoms. Most preferably the ethylenically unsaturated monomercomprises a carboxylic acid group or derivatives thereof resulting in anacylated polymer, specifically in an acetylated polyethylene orpolypropylene. Preferably, the carboxylic reactants are selected fromthe group consisting of acrylic, methacrylic, cinnamic, crotonic, andmaleic, fumaric, and itaconic reactants. Said functionalized polymerstypically comprise between 1 and 10 wt % of carboxylic reactant or more.The presence of such functionalization in the resin may substantiallyenhance the dispersability of the resin and/or allow a reduction offurther additives present for that purpose such as surfactants.

The polymeric resin has a density as measured according to ISO1183 inthe range from 860 to 930 kg/m³, preferably from 870 to 920 kg/m³, morepreferably from 875 to 910 kg/m³. The inventors identified thatpolyolefin resins with densities within said preferred ranges provide animproved balance between the mechanical properties of the compositearticle and the processability of the suspension, especially the driedsuspension during the process of the invention.

The polymeric resin is a semi-crystalline polyolefin having a peakmelting temperature in the range from 40 to 140° C. and a heat of fusionof at least 5 J/g, measured in accordance with ASTM E793 and ASTM E794,considering the second heating curve at a heating rate of 10 K/min, on adry sample. In a preferred embodiment of the present invention thepolymeric resin has a heat of fusion of at least 10 J/g, preferably atleast 15 J/g, more preferably at least 20 J/g, even more preferably atleast 30 J/g and most preferably at least 50 J/g. The inventorssurprisingly found that with the increase heat of fusion the compositesheet showed further improved ballistic performance such as back facedeformation and peel strength. The heat of fusion of the polymeric resinis not specifically limited by an upper value, other than thetheoretical maximum heat of fusion for a fully crystalline polyethyleneor polypropylene of about 300 J/g. The polymeric resin is asemi-crystalline product with a peak melting temperature in thespecified ranges. Accordingly is a reasonable upper limit for thepolymeric resin a heat of fusion of at most 200 J/g, preferably at most150 J/g. In another preferred embodiment, a peak melting temperature ofthe polymeric resin is in the range from 50 to 130° C., preferably inthe range from 60 to 120° C. Such preferred peak melting temperaturesprovide a more robust processing method to produce the composite sheetsin that the conditions for drying and/or compaction of the compositesheet do need less attention while composites with good properties areproduced. The polymeric resin may have more than one peak meltingtemperatures. In such case at least one of said melting temperaturesfalls within the above ranges. A second and/or further peak meltingtemperature of the polymeric resin may fall within or outside thetemperature ranges. Such may for example be the case when the polymericresin is a blend of polymers.

The polymeric resin may have a modulus that may vary in wide ranges. Alow modulus resin with for example a modulus of about 50 MPa, willprovide very flexible and thus comfortable armour for example forapplication in bullet resistant vests. A high modulus resin with forexample a modulus of about 500 MPa may provide armours with somestructural performance, e.g. a good ear to ear compression resistancefor combat helmets. Each application may have an optimum modulus for theresin, related to the specific demands during the use of theapplication.

The application of the suspension to the HPPE fibres may be done bymethods known in the art and may depend amongst others on the moment thesuspension is added to the fibres, the nature of the sheet, theconcentration and viscosity of the suspension. The suspension may forexample be applied to the fibres by spraying, dipping, brushing,transfer rolling or the like, especially depending on the intendedamount of polymeric resin present in the composite article of theinvention. The amount of suspension present in the sheet may vary widelyin function of the intended application of the composite sheet and canbe adjusted by the employed method but also the properties of thesuspension. For some applications, low amounts of highly concentratedsuspensions are employed to reduce the energy and time need for dryingthe impregnated sheet. For other applications a low concentrationsuspension may be advantageous for example to increase the wetting andimpregnation speed with low viscous suspensions. Last but not least thesuspension concentration and quantity should be chosen to provide acomposite sheet with the required amounts of polymeric resin present asa matrix material in said composite sheet. In a preferred embodimentsaid concentration of polymeric resin is at most 25 wt %, preferably atmost 20 wt %, even more preferably at most 18 wt % and most preferablyat most 16 wt %. In another preferred embodiment the concentration ofthe polymeric resin is between 1 and 25 wt %, preferably between 2 and20 wt %, most preferably between 4 and 18 wt %, whereby the weightpercentage is the weight of polymeric resin in the total weight of thecomposite sheet. In a further preferred embodiment the concentration ofpolymeric resin is at least 15 wt %, preferably at least 18 wt % andeven more preferably at least 20 wt %. In another preferred embodimentthe concentration of the polymeric resin is between 10 and 50 wt %,preferably between 15 and 40 wt %, most preferably between 18 wt % and30 wt %, Such preferred higher concentrations of polymeric resin mayhave the advantage of a providing composite sheets with higherconcentration while reducing the time and energy required for theremoval of the water from the composite sheets.

Once the polymeric aqueous suspension is applied to the HPPE fibres, theimpregnated fibre, preferably the assembly comprising the impregnatedfibres, is at least partially dried. Such drying step involves theremoval, e.g. the evaporation of at least a fraction of the waterpresent in the assembly. Preferably the majority, more preferablyessentially all water is removed during the drying step, optionally incombination with other components present in the impregnated assembledsheet. Drying, i.e. the removal of water from the suspension, may bedone by methods known in the art. Typically the evaporation of waterinvolves an increase of the temperatures of the sheet close to or abovethe boiling point of water. The temperature increase may be assisted orsubstituted by a reduction of the pressure and or combined with acontinuous refreshment of the surrounding atmosphere. Typical dryingconditions are temperatures of between 40 and 130° C., preferably 50 and120° C. Typical pressure during the drying process are between 10 and110 kPa, preferably between 20 and 100 kPa.

The process of the invention may optionally comprise a step wherein thecomposite sheet is heated to a temperature in the range from the meltingtemperature of the polymeric resin to 153° C., before, during and/orafter the partially drying of the sheet. Heating of the sheet may becarried out by keeping the sheet for a dwell time in an oven set at aheating temperature, subjecting the impregnated sheet to heat radiationor contacting the layer with a heating medium such as a heating fluid, aheated gas stream or a heated surface. Preferably, the temperature is atleast 2° C., preferably at least 5° C., most preferably at least 10° C.above the peak melting temperature of the polymeric resin. The uppertemperature is at most 153° C., preferably at most 150° C., morepreferably at most 145° C. and most preferably at most 140° C. The dwelltime is preferably between 2 and 100 seconds, more preferably between 3and 60 seconds, most preferably between 4 and 30 seconds. In a preferredembodiment, the heating of the sheet of this step overlaps, morepreferably is combined with the drying step. It may prove to bepractical to apply a temperature gradient to the impregnated sheetwhereby the temperature is raised from about room temperature to themaximum temperature of the heating step over a period of time wherebythe impregnated sheet will undergo a continuous process from drying ofthe suspension to at least partial melting of the polymeric resin.

In a further optional step of the process of the invention, thecomposite sheet is at least partially compacted by applying a pressure.Said pressure may be applied by compression means known in the art,which may amongst others be a calender, a smoothing unit, a double beltpress, or an alternating press. The compression means form a gap throughwhich the layer will be processed. Pressure for compaction generallyranges from 100 kPa to 10 MPa, preferably from 110 to 500 kPa. Thecompression is preferably performed after at least partially drying thecomposite sheet, more preferably during or after the optional step ofapplying a temperature, while the temperature of the sheet is in therange from the melting temperature of the polymeric resin to 153° C.

In a specific embodiment of the invention, a compression of thecomposite sheet may be achieved by placing the impregnated sheet duringor after the impregnation step or the partial drying step under tensionon a curved surface. The tension on that curved surface creates pressurebetween the fibers and surface. Filament winding is a well-knownproduction process for composites where this effect occurs, and it canadvantageously be applied in conjunction with the present invention.

The invention also relates to the composite sheet produced according tothe inventive process. Such composite comprises assembled HPPE fibresand a polymeric resin, wherein the polymeric resin is a homopolymer orcopolymer of ethylene and/or propylene, wherein the polymeric resin hasa density as measured according to ISO1183 in the range from 860 to 930kg/m³, a melting temperature in the range from 40 to 140° C. and a heatof fusion of at least 5 J/g. Such composite sheet is subject to thepreferred embodiments and potential advantages as discussed above orbelow in respect of the present inventive method, whereas the preferredembodiments for the composite potentially apply vice versa for theinventive method.

Preferably, the composite sheet comprises at least one network of thefibres. By network is meant that the fibres are arranged inconfigurations of various types, e.g. a knitted or woven fabric, anon-woven fabric with a random or ordered orientation of the fibres, aparallel array arrangement also known as unidirectional UD arrangement,layered or formed into a fabric by any of a variety of conventionaltechniques. Preferably, said sheets comprise at least one network ofsaid fibres. More preferably, said sheets comprise a plurality ofnetworks of the fibres. Such networks can be comprised in cut resistantgarments, e.g. gloves and also in anti-ballistic products, e.g.ballistic resistant articles, vests, helmets, radomes and tarpaulin.Therefore, the invention also relates to such articles.

A preferred embodiment of the present invention concerns a compositesheet containing more than 75 wt % of UHMWPE, preferably more than 80 wt% of UHMWPE and most preferably more than 85 wt % UHMWPE, whereby the wt% are expressed as mass of UHMWPE to the total mass of the compositesheet. In a yet preferred embodiment, the UHMWPE present in thecomposite sheet is comprised in the HPPE fibres of said composite sheet.

In a preferred embodiment, the composite sheet contains at least onemono-layer made according to the inventive process. The term mono-layerrefers to a layer of fibres. In a further preferred embodiment, themono-layer is a unidirectional mono-layer. The term unidirectionalmono-layer refers to a layer of unidirectionally oriented fibres, i.e.fibres that are essentially oriented in parallel. In a yet furtherpreferred embodiment, the composite sheet is multi-layered compositesheet, containing a plurality of unidirectional mono-layers thedirection of the fibres in each mono-layer preferably being rotated witha certain angle with respect to the direction of the fibres in anadjacent mono-layer. Preferably, the angle is at least 30°, morepreferably at least 45°, even more preferably at least 75°, mostpreferably the angle is about 90°. Multi-layered composite articlesproved very useful in ballistic applications, e.g. body armor, helmets,hard and flexible shield panels, panels for vehicle armouring and thelike. Therefore, the invention also relates to ballistic-resistantarticles as the ones enumerated hereinabove containing the inventivecomposite sheets. Preferably the sheet formed by aggregation of HPPEfibers is selected from the list consisting of a woven fabric, anon-woven fabric, a knitted fabric, a layer of unidirectional orientedfibres, a cross-ply of unidirectional oriented fibres or combinationthereof.

Another embodiment of the invention relates to a composite sheet, whichmay be used as a ballistic resistant sheet, comprising at least one,preferably at least 2, monolayers comprised of unidirectionally (UD)oriented fibers and the polymeric resin. Preferably the fiber directionin each monolayer being rotated with respect to the fiber direction inan adjacent monolayer. Several monolayers may be preassembled beforetheir use as ballistic resistant sheet. For that purpose a set of 2, 4,6, 8 or 10 monolayers may be stacked such that the fiber direction ineach monolayer is rotated with respect to the fiber direction in anadjacent monolayer, followed by consolidation. Consolidation may be doneby the use of pressure and temperature to form a preassembled sheet, orsub-sheet. Pressure for consolidation generally ranges from 1-100 barwhile temperature during consolidation typically is in the range from 60to 140° C.

The composite sheet may furthermore comprise a so-called separatingfilm, or cover sheet, being a polymeric film with a thickness ofpreferably from 1 to 20 micrometer, more preferably from 2 to 10micrometer. The separating film may comprise polyethylene, especiallyultra high molecular weight polyethylene, low density polyethylene,polypropylene, thermoplastic polyester or polycarbonate. Mostpreferably, biaxially-oriented films made from polyethylene,polypropylene, polyethylene terephthalate or polycarbonate are used asseparating films. Preferably separating films are employed incombination with low modulus resins for composite sheets in softballistic applications.

In a preferred embodiment, the weight, or areal density, of thecomposite sheet comprising at least one UD monolayer, including theweight of the fibers and matrix material is typically at least 25 g/m²,sometimes from 30 to 300 g/m², such as from 30 to 280 g/m². According tosome embodiments, the weight or areal density of the monolayer is from40 to 150 g/m².

The composite sheet of the invention is very suitable for use in softballistic articles, such as bullet-resistant vests. An alternative useof the composite sheet of the invention is in compressed or mouldedballistic resistant articles such as panels and especially curved panelsand articles, e.g. inserts, helmets, radomes.

The application of an aqueous polymeric suspension whereby the polymericresin present in said suspension is according to above describedembodiments is providing products with improved properties. The use ofan aqueous suspension of a polymeric resin as a binder material for HPPEfibres wherein the polymeric resin is a homopolymer or copolymer ofethylene and/or propylene, wherein the polymeric resin has a density asmeasured according to ISO1183 in the range from 860 to 930 kg/m³, a peakmelting temperature in the range from 40 to 140° C. and a heat of fusionof at least 5 J/g is hence a further embodiment of the presentinvention.

It is important that the polyolefin resin of the suspension softens ormelts at higher temperatures. So far such suspensions have not yet beenapplied in combination with HPPE fibres. Surprisingly, they provideimproved performance in various products especially products comprisingoriented UHMWPE fibres.

The combination of an oriented HPPE fibre with polyolefin polymers isdescribed in EP2488364 where melting of the polyolefin polymer isemployed to provide a flexible but strong sheets. However such productscontain substantial amounts of polyolefin resin or provide an inadequatewetting/distribution of the resin throughout the HPPE structure.Products such as described in EP2488364 are substantially different fromthe ones prepared according to the method according to the presentinvention, amongst others because in the currently presented methods andproducts the distribution of the polymeric resin is throughout thesheets providing improved mechanical properties. Furthermore theimpregnation of the HPPE fibre structure takes place at substantiallylower temperatures and in the absence of hydrocarbon solvents which mayavoid alterations of the HPPE fibres and/or their surfaces. Afterimpregnation, the water is removed and the remainder of the suspensionis present in a lower amount. The suspension may contain at least onesurface active ingredient such as ionic or non-ionic surfactant.

Sheets comprising HPPE fibres coated with a polymer having ethylene orpropylene crystallinity are also described in EP0091547, whereby mono-or multifilament fibers are treated at high temperatures with solutionsof the polymer in hydrocarbon solvents at a concentration of up to 12g/L. However, through such hot solvent treatment, the fibers may containresidual amounts of the employed hydrocarbon solvent negativelyaffecting fiber properties. Furthermore the treatment of the HPPE fiberat high temperature with a hydrocarbon solvent may affect structuralproperties of the fibers, especially through diffusion of thehydrocarbon solvent and/or polymer into the HPPE filaments. Thefiber-polymer interface may be modified by partial etching anddissolution of the HPPE which may affected amongst others the interfaceas well as the bulk properties of the HPPE fibers. In contrast, thepresent process may be performed at room temperature and employs anon-solvent for the HPPE, i.e. water. Accordingly the fibers andcomposite sheets produced by the process of the present invention mayhave a better retention of the structural properties of the HPPE fibers.The fibers may also present a different surface structure amongst whicha better discerned HPPE-coating interfaces compared to the fiberstreated at high temperature with a hydrocarbon solvent since nohydrocarbon solvent and/or polymer may diffuse into the HPPE fiber.Furthermore the process and products described in EP0091547 are limitedby the amount of polymer present in the hydrocarbon solutions and henceapplied to the HPPE fibers. The solutions are limited by theirincreasing viscosities and high amounts of polymer coating may only beapplied by repetition of the coating operation.

A preferred field of application of the composite sheet of the inventionis in the field of anti-ballistic articles such as armours. The functionof an armour is two-fold, it should stop fast projectiles, and it shoulddo so with a minimum deformation or size of the impact dent. It wassurprisingly observed that the size of the impact dent is small, ifcomposite sheets made according to the present invention are used inarmour. In other words, the back face signature is small. Such armour isespecially suitable for combat helmet shells, because they show reducedback face signature on stopping projectiles, thus reducing trauma on thehuman skull and brain after being hit by a stopped projectile.

The invention will be further explained by the following examples andcomparative experiment, however first the methods used in determiningthe various parameters useful in defining the present invention arehereinafter presented.

Methods

-   -   Dtex: yarn's or filament's titer was measured by weighing 100        meters of yarn or filament, respectively. The dtex of the yarn        or filament was calculated by dividing the weight (expressed in        milligrams) to 10;    -   Heat of fusion and peak melting temperature have been measured        according to standard DSC methods ASTM E 794 and ASTM E 793        respectively at a heating rate of 10K/min for the second heating        curve and performed under nitrogen on a dehydrated sample.    -   The density of the polymeric resin is measured according to ISO        1183.    -   IV: the Intrinsic Viscosity is determined according to method        ASTM D1601(2004) at 135° C. in decalin, the dissolution time        being 16 hours, with BHT (Butylated Hydroxy Toluene) as        anti-oxidant in an amount of 2 g/l solution, by extrapolating        the viscosity as measured at different concentrations to zero        concentration.    -   Tensile properties of HPPE fibers: tensile strength (or        strength) and tensile modulus (or modulus) are defined and        determined on multifilament yarns as specified in ASTM D885M,        using a nominal gauge length of the fibre of 500 mm, a crosshead        speed of 50%/min and Instron 2714 clamps, of type “Fibre Grip        D5618C”. On the basis of the measured stress-strain curve the        modulus is determined as the gradient between 0.3 and 1% strain.        For calculation of the modulus and strength, the tensile forces        measured are divided by the titre, as determined above; values        in GPa are calculated assuming a density of 0.97 g/cm³ for the        HPPE.    -   Tensile properties of fibers having a tape-like shape: tensile        strength, tensile modulus and elongation at break are defined        and determined at 25° C. on tapes of a width of 2 mm as        specified in ASTM D882, using a nominal gauge length of the tape        of 440 mm, a crosshead speed of 50 mm/min.    -   Tensile strength and tensile modulus at break of the polyolefin        resin were measured according ISO 527-2.    -   Number of olefinic branches per thousand carbon atoms was        determined by FTIR on a 2 mm thick compression moulded film by        quantifying the absorption at 1375 cm−1 using a calibration        curve based on NMR measurements as in e.g. EP 0 269 151 (in        particular pg. 4 thereof).    -   Areal density (AD) of a panel or sheet was determined by        measuring the weight of a sample of preferably 0.4 m×0.4 m with        an error of 0.1 g. The areal density of a tape was determined by        measuring the weight of a sample of preferably 1.0 m×0.1 m with        an error of 0.1 g.

Materials

Suspension 1 was purchased from Dow Chemical company under the tradename HYPOD1000 and is a 56 wt % polyolefin aqueous suspension withmelting peaks at 51° C. and 139° C. and a heat of fusion of 28 J/g.

Suspension 2 was purchased from Michelman under the trade name ofMichem® Prime 5931 and is a 28 wt % suspension of an acrylate modifiedpolyolefin (with a melting peak at 78° C. and a heat of fusion of 29J/g) in water.

Suspension 3 was produced by extruding a mixture a plastomer (Queo 0210,commercially available from Borealis, with a density of 0.902 g/cm³, apeak melting point of 95° C. and a heat of fusion of 120 J/g) and asurfactant (Synperonic® F 108 purchased from SIGMA-ALDRICH) in a weightratio of 7 to 3 at 100° C. under addition of water. The resin content inthe suspension was determined to be 40 wt %.

-   Suspension 4 was a commercially available polyurethane suspension in    water.

EXAMPLES 1 TO 3 AND COMPARATIVE EXPERIMENTS A AND B

Oriented UHMWPE tape was produced according to the solid state powderprocess of EP1627719. The tapes of a thickness of 65 μm were slit alongtheir orientation (drawing) direction to a width of 20 mm. 2 tapes of20×200 mm² (12) and a rectangular piece of 10×20 mm² (14) were prepared(FIG. 1A) respecting the tape orientations (122) and (124) as depicted,for later assembly as shown in FIG. 1B. Assembly is such that theorientation direction (142) of the rectangular piece (14) isperpendicular to orientation direction (122) of the other 2 tapes (12).This difference of orientation direction is chosen, becauseperpendicular stacking is typical for armour and provides more criticaladhesion values as compared to specimens with aligned orientations.

Test sampled were prepared by brush coating the future contact surfacesof the tape samples (12) and (14) with suspensions 1 to 3, substantiallyevaporating the water from the suspension under ambient conditionsduring 20 minutes followed by assembling the individual pieces accordingto FIG. 1b and pressing the contact area with a flat metal plate at 139°C. and a mass of 5 kg for 30 seconds. Two Comparative Experiments A andB were prepared in an identical way with Suspension 4 and no suspensionrespectively.

The obtained test samples tested at room temperature and at 70° C. andwere clamped in a Zwick Z010 testing machine and loaded till fracture inthe direction of the orientation direction of the tapes (122). Thesamples without suspension failed during the careful clamping operationand could not be tested.

Fracture force [N] Fracture force [N] Suspension 23° C. 70° C. Example 11  98 n.a. Example 2 2 275 273 Example 3 3 359 294 Comp. Exp. A 4  34 22 Comp. Exp. B n.a. Not measurable Not measurableThe samples prepared with the polyolefin suspension show a substantiallyimproved shear strength as compared to the test sample with the PUR orwithout suspension.

EXAMPLE 4

A fibrous armor sheet material was made by impregnating a unidirectionallayer of Dyneema® 1760 SK76 fibers with a polyolefin suspension preparedby blending suspension 1 in a a 1:1 ratio with water. After drying theaerial density of the unidirectional layer was 65 g/m² with a fiber toresin ratio of 82:18. Four such unidirectional layers were cross pliedin a 0° 90° 0° 90° sequence and consolidated for 30 seconds at apressure of 30 bar and a temperature of 115° C. The resulting crossplied sheet, bare of further protective films, had an areal density of260 g/m². The sheets were very robust and allowed easy handling andstacking for producing hard ballistic armors, plates or helmets.

EXAMPLE 5

A stack of 28 sheets from Example 4 was made and pressed into a helmetshape in a deep draw mold with a gap of 7 mm. Pressing was performedduring 30 minutes at 165 bar and 135° C. The pressure was maintainedduring cooling until the temperature was below 80° C. The helmets weretrimmed to shape and subjected to shooting tests with 9 mm Parabellumbullets with a speed of 430 meters per second. The tests were performedwith the NIJ Ballistic Penetration test Headform, Model 100_00_HNMEaccording to N.I.J. 0106.01 standard for ballistic penetration testsusing Herbin Sueu Plastiline clay. Two helmets were made and each helmetwas subjected to four shots. The average depth of the four shots of eachhelmet was determined.

Comparative Experiment C:

2 further helmets were produced and tested according to examples 5 withthe difference that Dyneema® HB26, available from DSM Dyneema, was used.HB26 has an areal density of 264 g/m², with a polyurethane matrixcontent of about 18 wt %.

The shooting trauma depth were measured in the clay. The results arepresented in the table below for each helmet:

Trauma depth Example 5′ 12.6 mm Example 5″ 13.0 mm Comp. Ex. C′ 14.8 mmComp. Ex. C″ 13.6 mmThe trauma depths of the helmets of Example 5 are smaller than the onesof the comparative experiment C. The average observed reduction of about1.4 mm trauma depth is a significant improvement for combat helmets.

EXAMPLE 6

The tapes of Examples 1 where cut to square pieces of 40×40 cm². Thetapes were wetted by spraying them with about 40 ml/m² of suspension 1diluted with water to a solid content of 4 wt %. 74 tapes were dried andstacked in an alternating 0° 90° sequence to a total areal density of4.89 kg/m². The stacks were pressed during 45 minutes at a temperatureof 120° C. and a pressure of 165 bar. The stacks were cooled underpressure until a temperature of 80° C. was reached and then removed fromthe press.

Comparative Experiment D:

Example 6 was repeated by stacking 75 tape without the dilutedsuspension 1, resulting in a compressed stack with an areal density of4.89 kg/m². The compressed stacks of example 6 and ComparativeExperiment D were subjected to shooting tests with 1.1 gram FragmentSimulating Projectiles. The speed of the projectiles was chosen suchthat a part of them perforated and a part of them were stopped, thusmeasuring in the range of the critical speed. The actual speed of thestopped projectiles was recorded and the width of the delaminated areaof the corresponding stop locations was measured. The average stoppingspeeds and trauma width are presented in the table below:

Comparative Example 6 Experiment D Average stopping speed [m/sec] 451463 Average trauma width [mm] 81.5 89.4 trauma width/stopping speed[msec] 0.180 0.193

The difference in stopping speed was small and probably not ofstatistical significance. However, the difference in trauma width issignificant. Even after normalizing against the stopping speed, thetrauma in the panels according to the invention is smaller. Reduction intrauma width in armour plates is important in view of multi-hitperformance since small trauma width reduces the chance of a second hitto arrive at a pre-damaged location. Coherence of the armour platesafter being hit is better.

EXAMPLE 7

A Dyneema® fabric with an aerial density of 163 gram/m² was wetted witha polyolefin suspension 1. A black die was added to the suspensionbefore applying it to the fabric. After drying, a robust water tightflexible sheet was obtained with good impregnation of the fibers. Thefibre to polyolefin ratio of the sheet was 0.88 (47 wt % fibers). Across-sectional inspection of the impregnated fabric confirmed that thedyed polyolefin suspension was present throughout the fabric andespecially throughout the yarn bundels. Impregnating fabrics oforientated UHMWPE achieve good impregnation at high fiber content andthus good mechanical properties at low aerial density.

Comparative Experiment E

A flexible tarpaulin sheet made from the same Dyneema® fabric as used inExample 7, but employing a melt impregnation process method as describedin WO201104321. The fibre to polyolefin ratio of the sheet was 0.37 (27wt % fibers). A cross-sectional inspection of the impregnated fabric Eshowed substantial amount of voids and inhomogeneous impregnation of thefabric.

EXAMPLE 8

Flexible fibrous armor sheet material was made by impregnating aunidirectional layer of Dyneema® 1760 SK76 fibers with a 1:1 dilution ofsuspension 1 with water. After drying the aerial density of theunidirectional layer was 33 g/m² with a fiber to resin ratio of 82:18.The unidirectional layers were cross plied in a 0° 90° 0° 90° sequence,sandwiched between two low density polyethylene foils with a thicknessof 7 micrometer and compression molded for 2 minutes at a pressure of 30bar and a temperature of 115° C. Flexible sheet for soft ballisticapplications with an areal density of 146 g/m² were obtained.

The flexible sheets were stacked to form a soft ballistic armour whichwas compared to similar ballistic armour but using commerciallyavailable Dyneema® SB21 armour sheets having a built up comparable tothe ones according to the invention with the difference that the matrixis a non-crystalline styrenic rubber system (Comparative Experiment F).

The ballistic performance of the stacks build from the sheets of Example8 and Comparative F proved to be equivalent. Nevertheless peel testsshowed that the peeling strength of the sheets according to example 8are substantially higher than those of SB21. Moreover, the inventivesheets of example 8 had lower scatter (standard deviation) of thepeeling strength. That means that the risk of local low bonding anddelamination is lower for the sheets according to the invention. Peeltests are performed by peeling upper and lower layers apart, such thatthe 2^(nd) and 3^(rd) layer are separated. The results of the peelingtests are below.

SB21 Example 8 (reference) Average Peel strength [N] 4.46 4.00 Standarddeviation [N] 1.1 2.07

EXAMPLE 9

A fibrous armor sheet material was made by impregnating a unidirectionallayer of Dyneema® 880 SK99 fibers with suspension 2. After drying theaerial density of the unidirectional layer was 33 g/m² with a fiber toresin ratio of 83:17. Two of such unidirectional layers were stacked ina 0° 90° sequence and laminated. Thus resulting in a cross ply laminatewith an aerial density of 66 g/m². The sheets were reasonably robust andallowed easy handling and stacking for producing hard ballistic armorplates.

Armor plates were produced by stacking above cross plies in such a waythat always 0° 90° sequences occurred. The stacks were made to a totalaerial density of 14 kg/m² These stacks were pressed at 165 Bar, at atemperature of 135° C. during 30 minutes. Subsequently they were cooledunder pressure to 80° C. before the press was opened. The obtainedpanels were cut to pieces of 0.2 m×0.2 m.

The specimens of 0.2 m×0.2 m were subjected to ballistic testing, byputting them in front of a hard steel plate with a thickness of 7 mmhaving a central hole with a diameter of 0.14 m. Subsequently. Thespecimens were shot with a Nato Ball DM111 (obtained from MetallwerkElisenhütte GmbH, Article number 231007) projectile at a speed of 840m/sec. All projectiles were stopped. The deformation of the specimenswere measured using a high speed camera at the back side where the armorwas allowed to deform through the hole in the steel plate. Additionally,the final displacement were measured after the tests.

Comparative Experiment F and G

The process of Example 9 was repeated with the sole difference that theunidirectional layers have been treated with two commercial coatingscomprising as a resin PUR and SEBS for Comparative Experiment F and Grespectively.

The results are summarized below. For example 9, and Comp Exp. G severalspecimen were tested and are reported separately. Beside the improvedtest results it was observed that the panels produced with sheetsaccording to the invention showed no delamination of the unperforatedsheets, whereas several panels of the comparative examples F and Gdelaminated and sheet edges were often pushed through the hole of thesteel plate.

Dynamic displacement Final displacement Material [mm] [mm] Example 9 41− 33 − 47-37 33 − 27 − 38-31 Comp Experiment F >100 >100 Comp ExperimentG 75 − 79 − 62- >100 62 − 73 − 55- >100

1. A method for manufacturing a composite sheet comprising highperformance polyethylene fibres and a polymeric resin comprising thesteps of a) providing high performance polyethylene (HPPE) fibres b)assembling the HPPE fibres to form a sheet c) applying an aqueoussuspension of the polymeric resin to the HPPE fibres before, during orafter assembling; d) at least partially drying the aqueous suspension ofthe polymeric resin applied in step c); to obtain a composite sheet uponcompletion of steps a), b), c) and d); e) optionally applying atemperature in the range from the melting temperature of the resin to153° C. to the sheet of step c) before, during and/or after step d) toat least partially melt the polymeric resin; and f) optionally applyinga pressure to the composite sheet before, during and/or after step e) toat least partially compact the composite sheet, wherein the polymericresin is a homopolymer or copolymer of ethylene and/or propylene andwherein said polymeric resin has a density as measured according toISO1183 in the range from 860 to 930 kg/m³, a peak melting temperaturein the range from 40 to 140° C. and a heat of fusion of at least 5 J/g.2. The method according to claim 1 wherein the HPPE fibres are selectedfrom the list consisting of tapes, filaments and staple fibres.
 3. Themethod of claim 1, wherein the HPPE fibres are prepared by a meltspinning process, a gel spinning process or solid state powdercompaction process.
 4. The method according to claim 1, wherein theconcentration of polymeric resin in the aqueous suspension is between 4and 60 wt %, preferably between 5 and 50 wt %, most preferably between 6and 40 wt %, whereby the weight percentage is the weight of polymericresin in the total weight of aqueous suspension.
 5. The method accordingto claim 1, wherein the HPPE fibres have a tenacity of at least 1.0N/tex, preferably 1.5 N/tex, more preferably at least 1.8 N/tex.
 6. Themethod according to claim 1, wherein the HPPE fibres comprise ultra highmolecular weight polyethylene (UHMWPE), preferably the HPPE fibressubstantially consist of UHMWPE.
 7. The method according to claim 1,wherein the amount of polymeric resin in the composite sheet is between1 and 25 wt %, preferably between 2 and 20 wt %, most preferably between4 and 18 wt %, whereby the weight percentage is the weight of polymericresin in the total weight of the composite sheet.
 8. The methodaccording to claim 1, wherein the density of the polymeric resin is inthe range from 870 to 920 kg/m³ preferably from 875 to 910 kg/m³.
 9. Themethod according to claim 1, wherein the peak melting temperature is inthe range from 50 to 130° C., preferably in the range from 60 to 120° C.10. The method according to claim 1, wherein the heat of fusion is atleast 10 J/g, preferably at least 15 J/g, more preferably at least 20J/g, even more preferably at least 30 J/g and most preferably at least50 J/g.
 11. A composite sheet obtainable by claim 1, comprisingassembled HPPE fibres and a polymeric resin, wherein the polymeric resinis a homopolymer or copolymer of ethylene and/or propylene, wherein thepolymeric resin has a density as measured according to ISO1183 in therange from 860 to 930 kg/m³, a melting temperature in the range from 40to 140° C. and a heat of fusion of at least 5 J/g.
 12. The compositesheet according to claim 11 wherein the sheet is selected from the listconsisting of a woven fabric, a non-woven fabric, a knitted fabric, alayer of unidirectional oriented fibres, a cross-ply of unidirectionaloriented fibres or combination thereof.
 13. The composite sheetaccording to claim 11, containing at least 75 wt % of UHMWPE, wherebythe weight percentage is the weight of UHMWPE in the total weight of thecomposite sheet.
 14. An article comprising the composite sheet accordingto claim 11, wherein the article is a ballistic resistant article, ahelmet, a radome or a tarpaulin.
 15. Use of an aqueous suspension of apolymeric resin as a binder material for HPPE fibres wherein thepolymeric resin is a homopolymer or copolymer of ethylene and/orpropylene, wherein the polymeric resin has a density as measuredaccording to ISO1183 in the range from 860 to 930 kg/m³, a peak meltingtemperature in the range from 40 to 140° C. and a heat of fusion of atleast 5 J/g.